Why won't my HTC M8 turn on?

I don't know why my phone won't turn on. I was using it and it was working perfectly fine... well, it was being a tad slow... I left it in bed fully charged for less than 3 minutes and when I came back it was off and hasn't turned back on. I really like this phone. I'm happy with it plus I have a lot of things there... Can anyone help please?